1. Field of the Invention
This invention relates generally to apparatus, such as a video tape recorder (VTR) of the so-called "helical-scan type", in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape and, more particularly, is directed to an improved tracking control system by which a magnetic head or other transducer in such apparatus is made to accurately scan the record track or tracks for reproducing the video or other information signals recorded therein.
2. Description of the Prior Art
In a helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and the transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During the recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. If the speed and direction of advancement of the tape are the same during a reproducing operation, as during a recording operation, then the scanning path of the head will be parallel to each record track and a servo system may control either the speed of tape advancement or the rotary speed of the head for obtaining proper scanning of the head along each track. However, if the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not coincide with a record track on the tape during each movement of the head across the tape and, accordingly the recorded video or other information signals may not be correctly or accurately reproduced.
Various tracking control or servo systems have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In the most desirable of these known arrangements, for example, as disclosed in U.S. Pat. Nos. 4,163,994, 4,172,264, 4,237,399, 4,287,538 and 4,296,443, each having a common assignee herewith, the head is mounted on the rotary drum portion by way of an electro-mechanical head deflecting device, such as, a bi-morph leaf, for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in respect to the direction along each of the record tracks. In the reproducing mode of the VTR, any deviation or tracking error of the head scanning path relative to a recorded track is detected and an electrical drive signal for the bi-morph leaf is correspondingly controlled to correct the tracking error with a view to obtaining a reproduced picture of high quality free of so-called guard band noise even in non-normal reproducing modes, such as, still, slow, fast or reverse reproducing modes.
In the non-normal or speed-varied reproducing modes, that is, when the speed and/or direction of advancement of the magnetic tape are not the same during reproducing as during recording, either a track is scanned a plurality of times in a so-called overlapped or repeat tracing for a slow-motion or still motion reproducing mode, or the tracks are only scanned at intervals in a so-called skip or interval tracing for a quick or fast motion reproducing mode. In the course of each of the foregoing non-normal reproducing modes, it is necessary that, at certain times, the playback head be made to jump or fly-back with a pitch corresponding to an integer or whole multiple of the track pitch between the terminal end of a track which has been scanned and the initial end of the track which is to be next scanned or traced.
Generally, it is desired to determine the mode of such head jump, which includes its direction and amount, so that there will be the smallest possible deflection of the bi-morph leaf from its neutral or zero position to cause the head to trace the initial end of the next track to be scanned. Moreover, the mode of the head jump before each trace is generally predetermined so that the maximum deflection of the bi-morph leaf required to make the head accurately scan the selected track at any time during such trace will not exceed a predetermined amount. For example, in published Japanese Patent Application No. 117,106/1977 (Applicant: Ampex Corporation), it is disclosed to control the head jump by detecting the drive voltage to the bi-morph leaf as an indication of the deflection of the head required for accurately scanning a record track thereby, and, when the detected voltage reaches a predetermined value, such as that at which the head deflection approaches, or would in the next trace approach the physical limit of that possible with the bi-morph leaf or other transducer deflecting device supporting the head, the drive voltage for the bi-morph leaf is suitably controlled to cause the mode of head jump required to select, as the next track to be scanned, one that avoids unacceptable deflection of the bi-morph leaf. However, in the foregoing arrangement, the tracing form or pattern of head jumps that is achieved will change undesirably when the bi-morph leaf or other head deflecting device becomes saturated or when its sensitivity, that is, the ratio of its deflection to a unit change in the drive voltage, or the linearity of its control circuit varies with time or age.
In order to avoid the foregoing problem, it has been proposed, for example, in U.S. Pat. Nos. 4,287,538 and 4,296,443, to control the mode of the head jump on the basis of a frequency and phase of the reproduced signal, for example, the frequency and phase of the reproduced sync signals, which change with movements of the tape relative to the head. Since the scanning direction of the head is skewed relative to the longitudinal direction of movement of the tape, displacement of the head by the bi-morph leaf or head deflecting device in the direction normal to the scanning direction has a component in the longitudinal direction of the tape, and hence is equivalent to a movement of the tape relative to the head so as to cause corresponding changes in the frequency and phase of the reproduced sync signals. It will be appreciated that such changes in the frequency and phase of the reproduced sync signals are dependent on the actual displacement of the head and are not influenced by the condition of the bi-morph leaf or of its control circuit. Therefore, at least in theory, controlling the mode of the head jump on the basis of the frequency and phase of the reproduced sync signals should provide stable and accurate head jump control. However, relatively complicated circuits are required for determining the desired mode of head jump on the basis of the frequency and phase of the reproduced sync signals so that the theoretically possible stability and accuracy may not be achieved. This is particularly true when analog data as to the frequency and phase of the reproduced sync signals are compared in respective comparators with external or reference sync signals, in which case the expected errors in analog processing and the instability due to temperature characteristics of the respective circuit elements may adversely affect the accuracy of head jump control.